#' @title Generate lighter or darker version of a color
#' @description Produces a linear blend of the color with white or black.
#' @param color_hex A character string representing a hex color
#' @param strength The "strength" of the blend with white or black,
#'   where 0 is entirely the original color and 1 is entirely white
#'   (`lighten_color()`) or black (`darken_color()`).
#' @examples
#' blue <- "#0e6ba8"
#' blue_light <- lighten_color(blue, strength = 0.33)
#' blue_dark <- darken_color(blue, strength = 0.33)
#'
#' if (requireNamespace("scales", quietly = TRUE)) {
#'   scales::show_col(c(blue_light, blue, blue_dark))
#' }
#'
#' @return A character string with the lightened or darkened color in
#'   hexadecimal format.
#' @name lighten_darken_color
NULL

#' @rdname lighten_darken_color
#' @export
lighten_color <- function(color_hex, strength = 0.7) {
  stopifnot(strength >= 0 && strength <= 1)
  color_rgb <- col2rgb(color_hex)[, 1]
  color_rgb <- (1 - strength) * color_rgb + strength * 255
  rgb(color_rgb[1], color_rgb[2], color_rgb[3], maxColorValue = 255)
}

#' @rdname lighten_darken_color
#' @export
darken_color <- function(color_hex, strength = 0.8) {
  stopifnot(strength >= 0 && strength <= 1)
  color_rgb <- col2rgb(color_hex)[, 1]
  color_rgb <- (1 - strength) * color_rgb
  rgb(color_rgb[1], color_rgb[2], color_rgb[3], maxColorValue = 255)
}

#' @title Add alpha to hex color
#' @description Applies alpha (or opacity) to a color in hexadecimal form by
#'   converting opacity in the `[0, 1]` range to hex in the `[0, 255]` range
#'   and appending to the hex color.
#' @inheritParams lighten_darken_color
#' @param opacity Desired opacity of the output color
#' @examples
#' blue <- "#0e6ba8"
#' blue_transparent <- apply_alpha(blue)
#'
#' if (requireNamespace("scales", quietly = TRUE)) {
#'   scales::show_col(c(blue, blue_transparent))
#' }
#'
#' @return A character string with added opacity level as hexadecimal characters.
#' @export
apply_alpha <- function(color_hex, opacity = 0.5) {
  paste0(color_hex, as.hexmode(round(255 * opacity, 0)))
}

adjust_value_color <- function(color_hex, strength = 0.5) {
  color_hsv <- rgb2hsv(col2rgb(color_hex))[, 1]
  color_hsv["v"] <- strength
  hsv(color_hsv[1], color_hsv[2], color_hsv[3])
}

#' Choose dark or light color
#'
#' Takes a color input as `x` and returns either the black or white color (or
#' expression) if dark or light text should be used over the input color for
#' best contrast. Follows W3C Recommendations.
#'
#' @references <https://stackoverflow.com/a/3943023/2022615>
#' @param x The background color (hex)
#' @param black Text or foreground color, e.g. "#222" or
#' `substitute(darken_color(x, 0.8))`, if black text provides the best contrast.
#' @param white Text or foreground color or expression, e.g. "#EEE" or
#' `substitute(lighten_color(x, 0.8))`, if white text provides the best contrast.
#' @examples
#' light_green <- "#c4d6b0"
#' contrast_green <- choose_dark_or_light(light_green)
#' dark_purple <- "#381d2a"
#' contrast_purple <- choose_dark_or_light(dark_purple)
#'
#' if (requireNamespace("scales", quietly = TRUE)) {
#'   scales::show_col(c(light_green, contrast_green, dark_purple, contrast_purple))
#' }
#'
#' @return The `black` color or `white` color according to which color provides
#'   the greates contrast with the input color.
#'
#' @export
choose_dark_or_light <- function(x, black = "#000000", white = "#FFFFFF") {
  if (is_light_color(x)) eval(black) else eval(white)
}

is_light_color <- function(x) {
  # this function returns TRUE if the given color
  # is light-colored and requires dark text
  color_rgb <- col2rgb(x)[, 1]
  # from https://stackoverflow.com/a/3943023/2022615
  color_rgb <- color_rgb / 255
  color_rgb[color_rgb <= 0.03928] <- color_rgb[color_rgb <= 0.03928] / 12.92
  color_rgb[color_rgb > 0.03928] <- ((color_rgb[color_rgb > 0.03928] + 0.055) / 1.055)^2.4
  lum <- t(c(0.2126, 0.7152, 0.0722)) %*% color_rgb
  lum[1, 1] > 0.179
}

prepare_colors <- function(colors = NULL) {
  if (is.null(colors) || length(colors) < 1) return(NULL)

  if (is.null(names(colors))) {
    stop(
      "`colors` must have names corresponding to valid CSS classes",
      call. = FALSE
    )
  }

  if (any(grepl("\\s", names(colors)))) {
    stop(
      "Color names in `colors` must be valid CSS classes",
      " and cannot contain spaces",
      call. = FALSE)
  }

  if (any(grepl("[^[:alpha:]_-]", names(colors)))) {
    stop("Color names in `colors` must be valid CSS classes", call. = FALSE)
  }

  whisker::iteratelist(colors, "color_name")
}

full_length_hex <- function(x) {
  varname <- substitute(x)
  bad_hex_msg <- str_wrap(
    "`", deparse(varname), "` is not a hexadecimal color: \"", x, "\". ",
    "theme_xaringan() requires colors to be specified in hexadecimal format.",
    " If you used valid CSS colors in your xaringan theme, please convert ",
    "these colors to hex format, e.g. \"#1a2b3c\"."
  )
  check_color_is_hex(x, stop, bad_hex_msg)
  x <- sub("^#", "", x)
  if (nchar(x) == 3) {
    x <- strsplit(x, character(0))[[1]]
    x <- rep(x, each = 2)
    x <- paste(x, collapse = "")
  }
  paste0("#", x)
}

check_color_is_hex <- function(
  color,
  throw = warning,
  msg = "{color} is not a hexadecimal color"
) {
  is_probably_hex <- grepl("^#", color) &&
    !grepl("[^#0-9a-fA-F]", color) &&
    nchar(sub("^#", "", color)) %in% c(3, 6)
  if (!is_probably_hex) {
    msg <- glue::glue(msg)
    if (!is.null(throw)) throw(str_wrap(msg), call. = FALSE)
  }
  is_probably_hex
}